Created attachment 3987[details]
Patch to implement enhancement
This patch adds support to the categorymembers/cm query for the cl_timestamp field in the categorylinks table (which roughly approximates when a page was added to a category). It allows retrieval of timestamp data for category members, and introduces an option to choose how to sort the query (two options corresponding to table indices: by sortkey, which is the current method, or by timestamp).

Fixed in r23459, but without the sort option. I don't even know if we have an index to sort by timestamps, and frankly, I don't see why it would be of any use. All r23459 is add the cmprop=timestamp option.